1. Field of the Invention
The present invention relates to an electropositive production well treating fluid used for the operations of drilling, well completion, well pulling, and well fracturing or increasing production, and to a method of preparing the same. More particularly, the present invention relates to a drilling fluid and well completion fluid used for drilling and well completion in petroleum industry.
2. Description of the Related Art
When production well treating fluid is used as a drilling fluid in drilling a well, the production well treating fluid always is faced with a problem on how to better solve a contradiction between the “stabilization of borehole wall” and/or the protection of oil-gas reservoir and “stabilization of drilling fluid properties”. Generally, a drilling fluid of an anionic system (electronegativity) is favorable for stabilizing the properties of drilling fluid itself, not for stabilizing the borehole wall and protecting the oil-gas reservoir. The electronegative clay mineral is easily swelled and dispersed into drilling fluid. Such swelling and dispersion are adverse to the stabilization of borehole wall. Alternatively, the drilling fluid system added with cationic compounds is favorable for stabilizing the borehole wall and the protecting of the oil-gas reservoir due to decreasing expansion and dispersion of electronegative clay mineral, but not for stabilizing the properties of drilling fluid itself.
For a long time, the problem of stabilizing a borehole wall, especially the problem of stabilizing a borehole wall in clay shale strata, is one of important reasons complicating downhole in drilling engineering. Especially since the 1980s, as exploration spreads towards new area areas, the strata met in drilling are increasingly complicated, the problem of borehole wall instability is increasingly serious. This sets higher requirements for borehole wall stability of drilling fluid.
Water-base drilling fluids used in current drilling process are mostly an electronegative dispersed system formed by dispersing clay particles in water, wherein the dispersion of clay particles depends on excess of negative charges presented in the crystal structure of clay particle itself. Now, during treating with the drilling fluid, treating agents used are almost all the anionic types that have very strong anion group itself. For example, in the case of dispersant and colloid-stabilizing agent and the like, the main action mechanism is namely in increasing negative charges of clay particles, intensifying hydration effect caused by negative charges and increasing the hydrated film thickness on clay particle surface to achieve the object of stabilizing colloid. Such kind of electronegative water-base drilling fluid system is undoubtedly disadvantageous for clay mineral in borehole wall and strata. This is because whatever factor is able to disperse clay particles of drilling fluid, it leads certainly to hydration, swelling and dispersion of clay mineral in the borehole wall and strata, results in losing the stability of the borehole wall and complicates cases such as slump of borehole wall and seizure of drilling tool and the like.
Therefore, in order to inhibit the dispersion of clay and stabilize the borehole wall a main measure that has to be taken generally is to add cationic substance, for example, inorganic salts of high valence metals such as iron chloride, aluminum chloride, ferric sulfate, hydroxyaluminum and the like; cationic organic substances such as cationic polyacrylamides, black organo-electropositive glues and inorgano-electropositive glues and the like into drilling fluids. They are able to neutralize negative charges of the clay surface, decrease ξ (zeta) potential of clay particles, decrease the hydrated film thickness, thus weaken the hydration effect of clay, being favorable for stabilizing borehole wall (see also U.S. Pat. Nos. 4,765,415, 4,959,164 and 5,196,401 and the like). However, these measures for stabilizing the borehole wall are disadvantageous to stabilizing the colloid of drilling fluid and cause increasing filtration loss of drilling fluid to a great extent. In order to increase colloid stabilization of drilling fluid, a great mount of an anionic stabilizer (fluid loss additive and dispersant) has to be added, but the addition of these treating agents will increase the negative charge in quantity. So drilling fluid stability and strata stability are always in contradiction to each other.
In the 1970's, there were no few of patents that recommend using an inert drilling fluid system. They utilize magnesium oxide and the like as water insoluble superfine powders to manufacture drilling fluid so as to achieve the object of no dispersion of strata clay and no pollution of production reservoir. However, it is relatively difficult to stabilize such kind of drilling fluid, as a superfine powder has to be used and also a great amount of water-soluble polymer has to be used in order to stabilize the drilling fluid through the principle of kinetics.
Before and after the 1980s, some treating agents having positive charges appear one after the other. Especially in recent years, after a mixed layered metal hydroxide compound having positive charges (i.e. an inorgano-electropositive gel or abbreviated as MMLHC) and black organo-electropositive gel are used in drilling fluid system, relatively great progresses have been made. Because such kind of electropositive gel granules and clay particles form a stable complex, by forming the complex by polarizing water molecule through particle electric field no destruction of colloid occurs in this drilling fluid system.
In order to meet the requirements of drilling engineering, improving the properties of drilling-fluid is favorable for stabilizing borehole wall and favorable for protecting oil-gas reservoir, researchers turn their attention to study cationic drilling fluid system.
China Patent CN 1144833A discloses a drilling fluid with zero electrokinetic potential. The drilling fluid is added with a charge neutralizing agent of clay in an amount of 0.1-10% by weight of the total drilling fluid, aiming to make the electrokinetic potential of drilling solids to be zero in the drilling fluid. However, in actual operation, it is very difficult to retain the electrokinetic potential of drilling solids to be zero constantly in the drilling fluid. Because the concentration of drilling solids is increased continuously with increasing drilling footage continuously in the drilling process, there is no way to determine whether the electrokinetic potential of drilling solids is zero in downhole, even if charge neutralizing agent is supplied continuously.
China Patent CN 1037852 discloses a cationic drilling fluid containing the following components: at least a macromolecule cationic polymer as a flocculating agent coated, at least a small molecule organic cationic compound as a stabilizer of clay shale, at least a viscosifier, at least a fluid loss additive and at least a alkalinity control additive. In the invention a macromolecule cationic polymer is used as a flocculating agent coated, a small molecule organic cationic compound is used as an inhibiting stabilizer of clay shale, aiming to purge drilling solids in the electronegative drilling fluid system to serve the action of increasing drilling rate and stabilizing borehole wall. The foothold of the invention still is based on the electronegative drilling fluid system, no changing occurs substantively in the drilling fluid system. Only if the flocculating agent coated and small molecule organic cationic compound are added in an amount enough high, the system can be retained to exhibit electropositivity. But such kind of electropositive system is so very unstable that the drilling requirement failures to be met.
U.S. Pat. No. 4,507,210 discloses a method of preventing swelling and dispersion of clay particles by using an optimal liquid-phase component. The inventor has found that some low swelling clay particles have “amphoteric” feature, i.e. zeta-potential of shale particles may be transformed from electronegativity to electropostivity regarding said specific clay particles under the condition of pH value higher than 12. Shale shows a cementation instability at the point near zero potential (pH value of surface charge being at the point with zero), being flocculated into greater particles, so filtration loss of the dispersed liquid phase of shale is maximized at the point of zero potential. Furthermore, the discovery has been used in drilling fluid, well completion fluid and well pulling fluid. However, the invention still is based on the electronegative drilling fluid system, and the alkalinity conditioning agent is used to adjust zeta potential of clay particles to make it higher than zero, wherein zeta potential is meant of potential of clay particles, not potential of drilling fluid. Moreover, the deficiency of the invention is that requirements for performance parameter of drilling fluid during drilling a well are not taken into account. The filtration loss of the drilling fluid entered reservoir should be controlled to the level less than 8-5 ml which is given by the standard criteria for protecting oil-gas reservoir, and pH value of drilling fluid should be kept between 7-9 (below 10). Therefore, the inventive method is very difficult to be realized for existing drilling fluid system.
Alternatively, it is known currently that the well completion fluid is still in an electronegative system and has a zeta potential in the range of −6 mv to −20 mv, which exhibits electronegativity not able to inhibit swelling and dispersion of clay in oil layer. Furthermore, the well completion fluid has different amount of solid particles with a solid-phase content generally higher than 3% by weight. When well completion fluid is contacted with oil layer, said fluid would damage the oil-gas reservoir in a certain extent because clay particles would be swelled and dispersed by electronegative system as described above. The solid particles have to cause blockage of oil layer in a certain extent and thus reduce the production of crude oil. Moreover, such well completion fluids cannot inhibit swelling and dispersion of clays in oil layer, so these result in increasing the clay content in well completion fluid and requiring for adding a great amount of water and dispersant, thus a great amount of well completion fluid is wasted and environment is polluted. Therefore, it is in great request to study and prepare a water-based well completion fluid that not only can solve the problem of solid particle blocking, but also inhibit effectively dispersion of clay and obtain a good result of protecting oil layer.